Abundant supplies of natural gas are in existence. Almost universally such supplies comprise mixtures of methane, ethane, natural gas liquids (C.sub.3 's and higher) and acid gases such as carbon dioxide and hydrogen sulfide. Nitrogen and inert gases are usually also present in varying amounts.
Carbon dioxide can be employed in tertiary oil recovery from older oil fields by pumping it into selected wells in a field and withdrawing mixtures of hydrocarbons dislodged or extracted from the field by the action of a miscible bank of carbon dioxide. In addition to the extracted crude oil, a gas stream is produced which contains major proportions of carbon dioxide and various hydrocarbons and usually also contains hydrogen sulfide. Because carbon dioxide is utilized in the tertiary recovery, it usually is economical to recover it for reinjection.
The most valuable supplies of natural gas are generally those which have high contents of methane and higher hydrocarbons and relatively low contents of acid gases. Such gases can be readily purified to pipeline quality by absorption of the acid gases in an amine solution which is regenerated and reused. Regeneration, however, requires considerable energy, and the amine absorption process accordingly becomes unsuitable because of high energy costs in cases where the gas mixture to be treated contains high proportions of acid gases.
For gas mixtures having acid gas contents higher than about 20 mol percent, it generally is more economical to utilize other separation systems such as membrane permeation or cryogenic fractionation. In a conventional cryogenic system utilized for purification of a feed gas having about 40 mol percent carbon dioxide, the chilled feed gas is introduced into a demethanizer column maintained at a pressure of about 650 psia at an intermediate point. The overhead distillate leaves the column at a temperature of about minus 37.degree. F. and is refrigerated to minus 75.degree. F. The condensed liquid is reintroduced as reflux at the top of the column. The overhead vapor which contains about 14 mol percent carbon dioxide is subjected to amine absorption to remove the carbon dioxide, producing pipeline quality methane. The reason for "slopping" about 14 percent of the carbon dioxide in the demethanizer overhead is to keep the system temperatures above the carbon dioxide solidification point. If solid carbon dioxide is allowed to form, the system becomes inoperable due to plugging. A reboiler at the bottom of the column maintains the bottom temperature at about 64.degree. F. The bottoms stream is conducted to a carbon dioxide separation column. The overhead from this column is carbon dioxide product, and the bottoms from the column are sent to natural gas liquids fractionation apparatus.
Other arrangements for coping with the problem at carbon dioxide solidification have been proposed. U.S. Pat. No. 2,996,891 to Tung and U.S. Pat. No. 3,242,681 to Shaievitz disclose methods in which the carbon dioxide is purposely solidified and thereafter separated from the liquid methane. The separation of the solid carbon dioxide, however, is not a simple operation, particularly at high pressure and low temperature conditions.
U.S. Pat. No. 3,784,466 to Arnold et al discloses methods for treating oil and gas to stabilize the oil. The gas removed from the oil is contacted with a solvent which has a greater affinity for heavier hydrocarbons, acid gases and water than for methane, ethane, and propane. The solvents named are acetone, propylene carbonate, dimethyl ether, polyethylene glycol, n-methyl-2-pyrrolidone, and methanol. The solvent is utilized to scrub a stream of rising gases. U.S. Pat. No. 3,252,269 to Woertz also discloses the use of selective solvents (particularly acetoxyacetone in combination with a second solvent) to effect absorption from the gas to be purified.
A second problem which arises after methane is cryogenically separated from carbon dioxide is that ethane and hydrogen sulfide as well as heavier hydrocarbons are separated as bottoms from the column together with the carbon dioxide and their separation from the carbon dioxide to render a relatively pure pipeline quality carbon dioxide is difficult. Ethane azeotropes with carbon dioxide in a conventional distillation column and comes off at the top with the carbon dioxide product. Hydrogen sulfide has a volatility close to that of carbon dioxide and the CO.sub.2 product usually would fail to meet governmental requirements such as the Texas Railroad Commission Rule 36 limitation of 100 ppm of H.sub.2 S.